The present invention is directed to engine control systems that control engine air flow, and more particularly to a method and system that calculates air flow ratios rapidly to respond to rapid changes in throttle position.
In an internal combustion engine, it is important to monitor and control the mass air flow, or the amount of air flowing into the engine, to maintain an optimum air/fuel mixture. As is known in the art, there are many engine components and systems that affect the engine air flow, and nearly all of these components are controlled by a powertrain control module (PCM). The ratio of the current engine load (normalized airflow) to the maximum engine load at the current barometric pressure, or air flow ratio, is periodically calculated to allow robust actuator scheduling and estimation of vacuum-driven flows.
The air flow ratio calculation tends to be quite slow and detailed because it must take into account the effect of the many PCM-controlled components and systems on the engine air flow. But once the air flow ratio is calculated in this manner it is a very accurate indicator of the pressure ratio across the throttle. The throttle position also affects the air flow ratio, but it is not controlled by the PCM. As a result, rapid changes in the throttle position can cause the calculated air flow ratio to become inaccurate rather quickly, particularly when the throttle position changes faster than the air flow ratio calculation update rate. For example, the air flow ratio calculation rate may be as slow as one calculation per second, while the throttle position may change at a much faster rate, on the order of 1000 degrees per second. Faster calculations of the air flow ratio may be needed to, for example, control and estimate air flow through sharp-edged orifices into the engine manifold due to the rapid changes in throttle position.
Current methods are unable to take rapidly changing throttle positions into account when calculating the air flow ratio because the throttle position tends to change before the air flow ratio calculation for the previous throttle position is complete. Further, any attempts to increase the calculation rate to respond to rapid throttle position changes would add significant chronometric burden to the system.
There is a need for a method that takes rapid throttle position changes into account when calculating the air flow ratio while preserving chronometric efficiencies.
Accordingly, the present invention is directed to a method for updating the air flow ratio quickly to account for rapid changes in the throttle position. More particularly, the method includes calculating an approximate air flow ratio in addition to a normal, conventionally calculated air flow ratio. The approximate air flow ratio can be updated more quickly than the normal air flow ratio, allowing the approximate air flow ratio to reflect changes in the throttle position even if the throttle position is rapidly changing.
The inventive method includes setting a reference throttle position, calculating a normal air flow ratio, sampling a throttle position, and calculating a first throttle position load as a ratio between the sampled throttle position and the reference throttle position. Whenever an updated air flow ratio is needed (such as whenever the throttle changes position), the throttle position is sampled again to obtain a current throttle position. A second throttle position load is calculated as a ratio between the current throttle position and the reference throttle position. The approximate air flow ratio is then calculated for the new throttle position based on the previously calculated normal air flow ratio and the difference between the first and second throttle position loads.
Because the approximate air flow ratio is based only on changes in the throttle position, it can be re-calculated quickly each time the throttle moves. This allows the air flow ratio to always be at fairly accurate even if the throttle is moving rapidly. Further, if the throttle is not moving, the conventionally-calculated normal air flow ratio continues to be very accurate. As a result, the present invention provides at least an approximate, and possibly very accurate, air flow ratio regardless of the rate at which the throttle position changes.